The spontaneous spreading of a liquid on a solid surface, that is to say, the wetting of a solid surface by a liquid, has been studied extensively. See, for instance, "Contact Angle, Wettability, and Adhesion," Advances in Chemistry Series, Volume 43, American Chemical Society, Washington, D.C. 1964, especially pages 1 through 49. As discussed there, spontaneous spreading occurs if the contact angle between liquid and solid goes to zero, and conversely, spontaneous spreading does not occur if this contact angle remains finite. The latter occurs when, loosely speaking, the surface free energy of the solid/liquid interface is less than or equal to the difference in surface free energy between the solid/vapor interface and the liquid/vapor interface. This relationship suggests that liquids will tend not to spread spontaneously on solid surfaces of suitably low surface free energy.
This theoretical understanding of the conditions under which wetting of solid surfaces by liquids occurs has made it possible to devise methods for the prevention or reduction of the spreading of liquids on surfaces. These methods have been categorized and discussed by M. K. Bernett and W. A. Zisman (op. cit., Chapter 24, pages 332-340). A first approach consists of appropriate modification of the solid surface to lower its critical surface tension of wetting. This can be done by means of an appropriate low energy surface coating. This approach was used in the invention that forms the basis of the copending U.S. patent application, Ser. No. 261,890, filed May 8, 1981 by Schonhorn and L. H. Sharpe, entitled "Prevention of Surface Mass Migration by Means of a Polymeric Surface Coating."
A second approach for preventing or reducing the spreading of a liquid on a solid surface, also discussed by Bernett and Zisman (ibid.) consists of addition of a selected solute, to be referred to as an "inhibitor," to the liquid. If an additive, while dispersed or dissolved in a liquid, can adsorb on a solid surface in contact with the liquid and form a thin layer thereon, with the resulting coated surface having a critical surface tension of wetting that is lower than the surface tension of the additive-containing liquid, then the liquid will not spread on the surface, and the additive is a potential inhibitor.
In a variant of the inhibitor method, also discussed by Bernett and Zisman (op. cit.), the additive is more volatile than the liquid and creates a surface tension gradient at the edge of the liquid drop that opposes the spontaneous spreading of the liquid. This application is concerned with the inhibitor approach and does not require creation of a surface tension gradient. The application will, therefore, not further discuss the gradient variant.
Methods for preventing or reducing the spreading of liquids have in the past been developed for, and applied to, oils. The discussion by Bernett and Zisman, cited above, is an example of such work. These authors recite classes of additives they find to be useful for preventing the spreading of liquids, namely fluorocarbon or silicone derivatives, fatty acids or other paraffinic polar compounds, or branched-chain or cyclic hydrocarbon derivatives, depending on the surface tension of the liquid (ibid, pp. 334-335). Other publications, for instance, R. L. Cottington et al, (op. cit., pages 341-354), deal exclusively with the spreading of oils on solids. Cottington et al report that effective adsorbable additives for the prevention of spreading of oils include the silicones, fluoroesters, amine-organic acid salts, high molecular weight organic acids, alcohols, or amines, and some oil-soluble soaps (op. cit., page 341), and teaches that the additive should have appreciable solubility in the base oil over the temperature range contemplated, should adsorb promptly on the surface from the leading edge to give a film from which the oil retracts, that it should lower the surface tension of the base oil by less than 5 dynes/cm, that it should be more volatile than the base oil, and that it should not be altered by hydrolysis or oxidation in such a way as to increase the surface free energy of the adsorbed film. The prior art does not seem to have given consideration to inhibitors potentially capable of prevention of spreading of oils or other liquids at elevated temperatures.
Oils are not the only liquids whose spreading tendency is of technological concern. In the manufacture of semiconductor devices it is, for instance, found that conductive adhesives, used for bonding a semiconductor component, e.g., a chip, to, e.g., a metallized substrate, are subject to spreading. Such spreading can have quite deleterious results. For instance, it can result in coating of bonding pads used for connections to external circuitry and consequent reduction of the strength and reliability of wire connections made thereto, as well as in other difficulties in later processing steps due to surface contamination by the spreading liquid. Similarly, in hybrid integrated circuits the spreading of screen-printed liquid fine line patterns can be a problem. Also, the spreading of a transparent encapsulant of an electrooptical device, e.g., a LED chip, can, for instance, result in a change of curvature of the encapsulant surface, with attendant change of the optical properties of the device. These examples indicate the variety of situations in which the spreading of liquids can occur and have undesirable consequences.
A method for preventing spreading of liquid is disclosed in U.S. Pat. No. 4,143,456, issued Mar. 13, 1979 to K. Inoue, entitled "Semiconductor Device Insulation Method." The method comprises applying a low surface free energy insulating resin film around a semiconductor chip on a circuit board by means of a printing technique, thereby confining a liquid, e.g., an uncured resin, to the space defined by the printed pattern.
Another method for prevention of liquid spreading is disclosed in the copending application, cited above. The method comprises coating of the substrate with a thin, typically a monomolecular, layer of an organic polymer, the layer having a low critical surface tension of wetting. Both these prior art methods thus require an added manufacturing step, namely, coating of all or a selected region of the substrate. A method for the prevention of liquid spreading that does not require such additional manufacturing steps is thus of economic significance, especially a method capable of the prevention of spreading at elevated temperature.